fix TestRenterRenew
fix TestRenterRenew

Mining testPeriod blocks causes the test to fail because the renter
will be trying to renew contracts during the download. This also
occurs for testPeriod/2, but the test does not fail because of the
API polling immediately after the mining: the poll ends when the
contract ID changes, but when testPeriod blocks are mined, the
contract ID will have changed twice, not once.

MrStrong/Sia,NebulousLabs/Sia,ericflo/Sia,mharkus/Sia,mharkus/Sia,ChrisSchinnerl/Sia,mtlynch/Sia,ericflo/Sia,ChrisSchinnerl/Sia,mtlynch/Sia,MrStrong/Sia,NebulousLabs/Sia